To determine the structure of aqueous sodium hydroxide solutions, results obtained from x-ray diffraction and computer simulation (molecular dynamics and Car-Parrinello) have been compared. The capabilities and limitations of the methods in describing the solution structure are discussed. For the solutions studied, diffraction methods were found to perform very well in describing the hydration spheres of the sodium ion and yield structural information on the anion’s hydration structure. Classical molecular dynamics simulations were not able to correctly describe the bulk structure of these solutions. However, Car-Parrinello simulation proved to be a suitable tool in the detailed interpretation of the hydration sphere of ions and bulk structure of solutions. The results of Car-Parrinello simulations were compared with the findings of diffraction experiments.

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The complex dielectric permittivity of aqueous NaOH (c e 2 M) and of dilute (e0.6 M) NaAl(OH) 4 and NaB(OH) 4 solutions in NaOH ([Na] ) 1 M) at 25 °C has been determined in the frequency range 0.2 e ν/GHz e 20. All spectra could be represented by a single Cole-Cole relaxation time distribution attributed to the cooperative relaxation of the solvent. The concentration dependence of the effective hydration number of OHhas been determined. For aluminate and borate solutions the deduced parameters: effective conductivity κ e , static permittivity , relaxation time τ, and distribution parameter R suggest a 1:1 replacement of hydroxide by aluminate and borate, accompanied by a release of bound water. The lack of an ion-pair relaxation process despite notable ion association suggests that rapid proton exchange is important not only for the dynamics of OHbut also for Al(OH) 4and B(OH) 4 -.

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Alkaline solutions containing polyhydroxy carboxylates and Ca(II) are typical in cementitious radioactive waste repositories. Gluconate (Gluc(-)) is a structural and functional representative of these sugar carboxylates. In the current study, the structure and equilibria of complexes forming in such strongly alkaline solutions containing Ca(2+) and gluconate have been studied. It was found that Gluc(-) significantly increases the solubility of portlandite (Ca(OH)2(s)) under these conditions and Ca(2+) complexes of unexpectedly high stability are formed. The mononuclear (CaGluc(+) and [CaGlucOH](0)) complexes were found to be minor species, and predominant multinuclear complexes were identified. The formation of the neutral [Ca2Gluc(OH)3](0) (log β213 = 8.03) and [Ca3Gluc2(OH)4](0) (log β324 = 12.39) has been proven via H2/Pt-electrode potentiometric measurements and was confirmed via XAS, (1)H NMR, ESI-MS, conductometry, and freezing-point depression experiments. The binding sites of Gluc(-) were identified from multinuclear NMR measurements. Besides the carboxylate group, the O atoms on the second and third carbon atoms were proved to be the most probable sites for Ca(2+) binding. The suggested structure of the trinuclear complex was deduced from ab initio calculations. These observations are of relevance in the thermodynamic modeling of radioactive waste repositories, where the predominance of the binuclear Ca(2+) complex, which is a precursor of various high-stability ternary complexes with actinides, is demonstrated.

The absolute (dynamic) viscosities (η) and densities (ρ) of carbonate-free aqueous tetramethylammonium and alkali metal hydroxides have been determined up to saturation concentrations ([NaOH] ≤ 19.l M, [KOH] ≤ 14.1 M, [LiOH] ≤ 4.8 M, [CsOH] ≤ 14.8 M, and [(CH3)4NOH] ≤ 4.2 M) at 25.00 °C using a Ubbelohde viscometer and a vibrating tube densitometer, respectively. The viscosities are believed to be precise to within 0.1% and the densities to within 5 × 10-6 g cm-3. Densities of isoplethic MOH solutions increase in the order of (CH3)4N+ < Li+ < Na+ < K+ ≪ Cs+. Viscosities for [MOH] < 4 M solutions increase in the reverse order, but the viscosities of CsOH solutions become extremely large at very high concentrations. The shape of the density vs concentration function of (CH3)4NOH solutions is also quite different when compared with the alkali metal hydroxide solutions. Density data were fitted up to the highest concentrations using the Masson equation. Viscosity vs concentration functions are represented in the form of a fifth-order (empirical) polynomial.

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